1. Field of the Invention
The present invention relates to an inverter device using gate turn-off thyristors (which will be shortly referred to as "GTO") and, more particularly, to an inverter device which is constructed to have GTOs connected in parallel so as to have its capacity increased.
2. Description of the Prior Art
An inverter device having parallel-connected GTOs is disclosed, for example, by Takashi Tsuboi and Takamasa Hori in Hitachi Review Vol. 31 (1982), No. 1, pages 23-27, "GTO Inverter Controlled AC Traction Drives" and is shown in FIG. 6 of the same. In the circuit disclosed, for each arm, two GTOs are connected in parallel through a current balancing reactor (i.e., a current balancer) AB. Moreover, the respective nodes of the positive and negative arms in U, V and W phases provide the a.c. output terminals of that inverter device.
A leakage inductance exists between the two coils of that current balancing reactor AB. In this circuit construction, therefore, when the two GTOs of each arm are turned off by the influence of that leakage inductance, an excessive voltage is applied between the anodes and cathodes of those GTOs which leads to concern that the GTOs may be broken.
In order to suppress such excessive voltage, generally speaking, a snubber circuit is connected in parallel with a GTO. This snubber circuit is constructed by connecting a capacitor C in series with a parallel connection of a resistor R and a diode CR1, as is shown in FIG. 12.8 on page 227 of "SILICON CONTROLLED RECTIFIER MANUAL, THIRD EDITION", for example, which was published in 1964 by General Electric Company. However, the snubber circuit is intrinsically provided to suppress an excessive voltage due to the wiring inductance. In order to suppress the excessive voltage generated by the leakage inductance of a current balancing reactor, therefore, it is necessary to increase the capacity of the capacitor C of the snubber circuit to such a level as corresponds to the leakage inductance of the current balancing reactor. If this capacity of the capacitor C is increased, however, the energy to be consumed by the resistor R of the snubber circuit is increased to drop the efficiency of the inverter device as a whole. Still worse, the capacitor C and the resistor R of the snubber circuit have their sizes enlarged to enlarge the inverter device.